The present invention relates generally to the production of metallic aluminum from alumina (Al.sub.2 O.sub.3) and more particularly to a method and apparatus for electrolytically reducing alumina to aluminum.
For many decades, the principal commercial method employed for the electrolytic reduction of alumina to aluminum has been the Hall-Heroult Process. In this process, a molten electrolyte comprising sodium cryolite (Na.sub.3 AlF.sub.6) as the principal constituent is contained in a cell or pot, the interior of which is lined with carbon. A pool of molten aluminum rests on the bottom of the cell and forms the cathode for the cell, and consumable carbon anodes located above the electrolyte bath extend downwardly through the top of the bath. Alumina is introduced into the molten electrolyte bath wherein the alumina dissolves and a number of reactions occur, eventually producing molten aluminum which accumulates at the bottom of the cell and carbon dioxide and some carbon monoxide, from a side reaction, which are given off from the top of the cell.
A substantial amount of electric power is consumed in the process. In addition, there is a continual consumption of carbon anodes which requires continual adjustment of the carbon anode downwardly toward the cathode at the bottom of the cell to maintain a desired anode cathode distance (ACD) between the two electrodes. The larger the ACD, the greater the resistance between the electrodes and the more power consumed. On the other hand, the ACD cannot be too small because of the danger of short-circuiting between the molten aluminum cathode and the anodes due to agitation by electromagnetic forces in the molten metal. Also, there can be an increase in power consumption due to an increase in bath resistance caused by an increase in the concentration of carbon dioxide gas bubbles in the space between the anode and a cathode when the distance between the two electrodes is decreased. Typically, the ACD averages about 1.5-2.0 inches (38-51 mm) in the Hall-Heroult process.
Because the carbon anodes are consumed during the process, they must be replaced regularly, and this requires an anode manufacturing plant and continual handling of new anodes and old anode "butts" in the plant. Moreover, hot anodes in the cells are also subject to wasteful "air burn".
The life of the cell is limited by swelling of the carbon lining which is believed to be due in large part to the formation of a lamellar sodium compound in the carbon. The sodium comes from a side reaction on the carbon surface in contact with the sodium cryolite.
If alumina is introduced into the bath at too fast a rate, the alumina will be incompletely dissolved in the bath, and some of it will sink through the molten aluminum below the bath and form a "muck" on the carbon lining at the bottom of the cell. Once the alumina sinks under the molten aluminum, it is shielded from dissolution in the electrolyte bath and further accumulates on the bottom of the cell. Muck creates a high resistance between the metal pool and the carbon bottom lining to which the current flows. This results in high temperatures which produce inefficient or "sick" pots. A typical normal operating temperature in the Hall-Heroult Process is 950.degree.-1,000.degree. C. (1,742.degree.-1,832.degree. F.).
The sick pot problem is controlled by deliberately starving the pots so that they go on "anode effect" about once a day. Anode effect is a high voltage condition at the anode which generates carbon monoxide and carbon tetrafluoride (CF.sub.4). The anode effect is indicated by an incandescent light connected across the cell. The light goes on when the voltage across the cell increases from a normal 4.5 to 5 volts to as much as 60 volts. The anode effect is eliminated by stirring alumina into the bath. The costs of anode effect are energy inefficiency, an increase in pot temperature and an increase in carbon anode consumption.
Numerous attempts have been made to increase the energy efficiency of the alumina reduction process and to solve various operating problems. These attempts have included the development of non-consumable, dimensionally stable anodes. A process using a non-consumable anode produces oxygen rather than carbon dioxide, but this requires an additional thermodynamic potential of one volt which is a disadvantage. The principal advantages are elimination of the anode manufacturing plant with its mixing and pressing equipment and baking furnaces, the elimination of the purchase of carbon anode materials and the elimination of anode and butt handling in the reduction plant.
Non-consumable anodes are made of electrically conductive oxides or of cermets (mixtures of oxides and metallic particles). Examples of non-consumable anode materials are tin oxide (SnO.sub.2) and oxides of iron and nickel bonded with metallic iron and nickel. However, with the latter type of anode, the concentration of the alumina in the bath must be kept greater than about 40% of the alumina content at which the bath is saturated with alumina to prevent dissolution of the anode at a standard operating temperature of about 975.degree. C. (1,787.degree. F.).
Developmental work has also been conducted on cathodes covered with titanium diboride (TiB.sub.2). Titanium diboride is an electrical conductor, it is wet by aluminum, and it is resistant to attack by molten aluminum and by the cryolite electrolyte. A cathode covered with titanium diboride has a carbon interior and is dimensionally stable.
Withers et al. U.S. Pat. No. 4,338,177, in FIG. 12, discloses a process for the electrolytic reduction of alumina employing a heavy electrolyte bath having a density greater than that of molten aluminum which accumulates as a pool on the top of the bath. A titanium diboride cathode extends downwardly through the top of the bath and terminates a relatively short distance from the bottom of the cell at which is located an anode comprising lumps or pieces composed of alumina and a carbonaceous material. The anode pieces reside at the bottom of the cell as a result of having a density greater than that of the heavy electrolyte bath or else are retained at the bottom of the bath by a grate. Carbon dioxide is liberated during the process, and this is undesirable. In addition to increasing the resistance of the bath, the carbon dioxide reportedly caused a back reaction with the aluminum floating on the top of the bath, thereby decreasing the efficiency of the process. A trough was used at the top of the bath to keep the aluminum pool out of the path of the carbon dioxide liberated during the process.
To form the anode, it was necessary to subject the mixture of alumina and carbonaceous material to high pressure molding to form anode pieces of sufficient density to sink to the bottom of the bath. Absent this high density, it was necessary to employ a graphite grate to hold the anode pieces on the bottom of the cell.
A process employed in the electro-refining of metallic aluminum utilizes an electrolytic cell having an electrolyte bath heavier than molten aluminum which thus accumulates at the top of the cell. In this process, graphite current collectors extend downwardly into the refined pool of aluminum on the top of the electrolyte bath, and the refined aluminum serves as the cathode for the cell. The anode is located at the bottom of the cell and is composed of an aluminum-copper alloy which is denser than the electrolyte. The bottom of the cell is lined with carbon, and steel conductor bars extend into the carbon lining at the bottom of the cell for the purpose of conducting current thereto.